Development of a Transient CFD Model of an SI Fuel Injector
Abstract
As the use of fuel injection in spark ignition engines has increased, continuous refinements in the design of fuel injectors are needed in order to obtain lower engine emissions and increased performance. In this endeavor, computational fluid dynamics (CFD) has been used as a means of gaining an understanding of the flow through the injector, and also as a valuable tool in the design process. Most CFD models constructed to study injector flow utilize the standard k-ε turbulence model and perform steady state calculations with the fuel injector needle held in a fixed position. The objectives of this research were to determine the appropriate turbulence model for this flow situation and the accuracy of using a steady state analysis to simulate the transient flow in an operational fuel injector. An evaluation of three turbulence models was performed. The standard k-ε, along with the Renormalisation Group (RNG) and the Chen modifications of the k-ε. scheme were used to obtain steady state flow results for a fuel injector in the fully open position. Star-CD was used to perform the simulations of two fuel injectors containing Ford compound nozzles, which are specifically designed to generate turbulent flow just upstream of the injector exit. This comparison resulted in the determination of an appropriate turbulence model, which was then used in a transient CFD simulation of the injector. In addition to the transient simulation, which modeled the opening and closing processes, four steady state simulations were performed at different needle lift positions. The results obtained from these steady state simulations were compared to those from the transient simulation at the same needle lift positions. In all cases, the flow properties used for the comparisons were the fluid velocity, the turbulent kinetic energy, and the turbulent dissipation in the nozzle exit plane. This location was chosen because of its influence on the spray dispersion and droplet size distribution produced by the injector. From the turbulence model study it was determined that the Renormalisation group and Chen's modifications schemes were preferred over the standard k-ε scheme for predicting the turbulent flow properties. Comparisons between the transient and steady state simulation results clearly illustrated that the rapid movement of the pintle needle during the opening and closing processes greatly influences the flow at the injector exit. From these observations, it was determined that a fuel injector can operate entirely within the transient mode. Therefore, it was concluded that a transient simulation is the preferred method to use for injector analysis.